記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.94.232

CiNii Research

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担当区分：筆頭著者,　責任著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.94.304

CiNii Research

その他リンク： https://www.jstage.jst.go.jp/article/jjws/94/5/94_304/_pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）

Creep-fatigue interaction is identified as a primary failure mode for components operating under high temperatures. As operational durations extend, this interaction not only alters the material's microstructures but also initiates a gradual degradation in mechanical properties, significantly impacting its deformation and damage behaviors. In this work, the dynamic microstructural evolution of GH4169 superalloy during creep-fatigue was elucidated via qualitative characterization, and damage level evaluation method was subsequently developed by bridging microstructure degradation to mechanical property degradation. Creep-fatigue tests were performed at 650 °C with various tensile holding times and were interrupted at lifetime fractions of 10 %, 50 % and 80 % for further analysis and tensile evaluations. Results revealed that the prolonged exposure to holding times induced the coarsening of γ^″ precipitates alongside an increase in low-angle grain boundaries, culminating a reduction in creep-fatigue strength. The development of voids and cracks exacerbated the degradation of elongation, leading to a hybrid fracture mode encompassing both intergranular and transgranular cracking paths. Synthesizing microstructural evolutions to qualitatively categorize diverse degradation levels imparted a robust physical basis for damage evaluation. A mapping model was established to correlate the average kernel average misorientation (micro-degradation indicator) with the tensile plastic strain energy density (macro-degradation indicator). The damage level evaluation method was endowed with quantitative metrics utilizing this model, and its generality was additionally validated in P92 steel. This work offers an insight into the quantitative damage evaluations of creep-fatigue-induced degradations in materials, thereby providing a theoretical basis for the development of operation management and inspection plans of components.

DOI： 10.1016/j.ijplas.2024.104086

Scopus

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The effect of the combined addition of Zn and Ni on the mechanical properties and microstructures of an intermetallic compound (IMC) layer of dissimilar welds of Al alloy 5052 to low-carbon steel was systematically investigated. A maximum weld strength of 120 MPa was achieved with the combined addition of Zn and Ni, which was 193 % higher than that of the weld produced without the addition of Zn or Ni. The addition of Zn resulted in the formation of soft Fe–Zn IMCs in the IMC layer, which reduced the hardness of the IMC layer. Additionally, the combined addition of Zn and Ni promoted the formation of Fe–Zn IMCs, which further softened the IMC layer. Further softening of the IMC layer, reduction in the thickness of the IMC layer, and suppression of the formation of defects in the IMC layer, achieved by the combined addition of Zn and Ni, drastically improved the weld strength.

DOI： 10.1016/j.matdes.2024.113302

Scopus

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記述言語：日本語   掲載種別：研究論文（学術雑誌）

Laser powder bed fusion, an additive manufacturing process, is suited to fabricate the aluminum parts with complicated shapes for aerospace and automotive industries. Laser powder bed fusion of aluminum alloys often leaves internal defects in the parts, but formation mechanism of internal defects has not been fully understood. In this study, effect of laser irradiation condition on internal defect and microstructure of the parts was examined to obtain the fundamental knowledge of the defect formation during laser powder bed fusion of Al-10Si-Mg alloy. Density of the internal defects was high in the parts produced at the low and high energy densities, while there were almost no defects in the part produced at meddle (appropriate) energy density. The high energy density resulted in spherical defects like blowholes, while the low energy density caused polygonal shaped defects. Formation of the polygonal shaped defects could be explained by insufficient melting of powders at the low energy density because width of the melt pool was very narrow. The microstructure was coarser at the higher energy density. The tensile strength and elongation were strongly affected by the amount of internal defects in the part, rather than the microstructure.

DOI： 10.11283/jlwa.62.581

Scopus

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担当区分：筆頭著者,　責任著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.93.346

Scopus

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担当区分：筆頭著者,　責任著者   記述言語：中国語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.93.194

Scopus

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担当区分：筆頭著者,　責任著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.11263/jjws.93.258

Scopus

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

The dissimilar joining of Al/Cu and Al/Ni is required to reduce the weight of electronic and electrical devices. Ultrasonic spot welding (USW) is expected to be applied to dissimilar materials because of the low heat input during the joining process. However, to obtain high strength via USW, the relationship among the welding parameters, joining strength, and microstructure for different material combinations must be investigated. In this study, Al/Al, Al/Cu, and Al/Ni are joined via USW by systematically varying the welding parameters; subsequently, their tensile shear load and microstructure are investigated. Results show that the vibration amplitude and welding time impose greater effects on the joint strength than the welding pressure for all material combinations. High-strength joints that fractured at the Al upper plate are obtained by optimizing the welding parameters for each material combination. Al/Ni bonding had a narrower margin of welding time to obtain a higher tensile shear load compared with other material combinations. This is due to the high heating rate and maximum temperature of Al/Ni joints, which cause the excessive deformation of the Al upper plate during prolonged welding. Texture analysis showed that recrystallized grains with GOSS orientation was remained in the high-strength dissimilar joints.

DOI： 10.1016/j.jmrt.2023.09.094

Scopus

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担当区分：筆頭著者,　責任著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.92.429

Scopus

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

As for the cause of ennoblement of stainless steel in freshwater, it has been reported that metabolites of microorganisms accelerate the consumption of electrons at the cathode. However, the actual state of the microbiome that leads to the ennoblement and the dynamics of them associated with the formation of anodes and cathodes have not yet been clarified. In this study, an electrochemical cell with stainless steel as the working electrode and counter electrode was placed in a lab-scale continuous culture system. In this experimental system, open circuit potential measurement and potential/current measurement during polarization were conducted. Then NGS (Next generation sequencing) analysis of the microbiome on the test specimen was also performed. A rapid increase in current during polarization was observed only in the biotic condition. From the result of NGS, the dominant trend of microorganisms which have active electron-accepting functions was confirmed.

Scopus

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

In this study, effect of welding parameters on microstructure and mechanical properties of an ultrahigh strength steel (UHSS) friction stir lap welded with Co-based alloy tool was examined. Friction stir lap welding of the UHSS was successfully done at various welding parameters. A decrease in rotational speed decreased fraction of martensite and hardness in the stir zone. Tensile shear load of the weld increased with decreasing the rotational speed. In the welds produced at the high rotational speed, crack was initiated from the tip of the lapped interface and then propagated into the stir zone having high fraction of martensite, possibly resulting from the stress concentration. On the other hand, the welds produced at the low rotational speed failed in the softest region of the heat-affected zone. This study showed that the reduced brittleness of the stir zone arising from the low rotational speed was an effective strategy for the superior mechanical properties of the lap welds of the UHSS.

DOI： 10.1007/978-3-031-22661-8_9

Scopus

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Dissimilar welding of an aluminum (Al) alloy to steel is expected to be effective for weight reduction of automotive car bodies. In this study, an aluminum alloy 1050 sheet was welded to a nickel (Ni)-plated Fe sheet using a silicon (Si)-containing Al filler by gas tungsten arc welding. The effect of the simultaneous addition of Si and Ni on the Al/Fe joint strength associated with the microstructure of the intermetallic compound (IMC) layer was systematically examined. The addition of Si and Ni individually improved the Al/Fe joint strength by the reduction in the IMC layer thickness caused by the Si addition and the refinement of the IMC grains due to the Ni addition, respectively. The most effective improvement in the joint strength was achieved by the simultaneous addition of Si and Ni. The IMC layer at the Al/Fe interface with Si addition consisted of θ-Fe<inf>4</inf>Al<inf>13</inf>, η-Fe<inf>2</inf>Al<inf>5</inf>, and Al–Fe–Si ternary IMCs, while it changed to θ-Fe<inf>4</inf>Al<inf>13</inf>, η-Fe<inf>2</inf>Al<inf>5</inf>, and an Al–Fe–Ni ternary IMC after Ni was additionally added. This change in the phase as well as the thickness reduction and grain refinement of the IMC layer probably contributes to further improving the joint strength by the simultaneous addition of Si and Ni.

DOI： 10.1016/j.matdes.2022.111444

Scopus

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記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

To secure the safety during decommissioning of Fukushima nuclear power plant, temporary repair of rusted steel structures with cracks, possibly located in water, is needed. Friction stir welding (FSW) could be used to repair these structures in water without the elimination process of rust layers, but availability of underwater FSW to rusted steels has hardly been examined. To clarify effect of underwater operation and rust layer on weldability of 0.45%C steel, rusted steel was welded by conventional and underwater FSW at the various rotational speeds, and then the thermal histories, FSW machine outputs, and microstructure and mechanical properties of the stir zone were examined. Underwater FSW could produce the sound welds with no welding defects on 0.45%C steel, but the range of the rotational speed for the sound welds was slightly narrow during underwater FSW. Underwater FSW exhibited the lower maximum temperature and the much higher cooling rate than conventional FSW, resulting in the higher hardness and tensile strength of the stir zone through formation of the microstructure including bainite. Moreover, the rusted steel could be successfully welded by conventional and underwater FSW. The rust layer, about 50 μm in thickness, hardly affected the weldability, and microstructure and hardness distributions of the weld. However, the fragmented rust was trapped and left near the top surface of the stir zone at the higher rotational speed, resulting in a decrease in strength of the stir zone.

DOI： 10.2355/tetsutohagane.TETSU-2022-024

Scopus

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A single-bead multilayer martensitic stainless steel thin wall was fabricated using the wire arc additive manufacturing (WAAM) process. After the WAAM process, the tempering heat treatment was carried out to homogenize the microstructure distribution in the wire arc additive manufactured (WAAMed) part. To optimize the tempering temperature, the as-WAAMed samples were subjected to varying tempering temperatures of 300, 500, 600, and 700 °C for 2 h. The heterogeneous structure within the as-WAAMed part was hardly eliminated by the low tempering temperatures of 300 and 500 °C. On the other hand, the inhomogeneous hardness profiles were significantly homogenized by the tempering temperature of 600 and 700 °C. Tempering at 600 °C removed the heterogeneous structure, resulting in the effective homogenization of the significantly heterogeneous hardness profile and microstructure distribution in the as-WAAMed part. Tempering at 700 °C led to the lower hardness than that at 600 °C.The anisotropic tensile properties of the as-WAAMed part were also eliminated after tempering at 600 °C. The significant differences in the toughness of the different layers of the as-WAAMed part were effectively reduced by proper tempering. The slight decrease in toughness of the post-process heat-treated sample from the bottom to the top of the thin wall was mainly caused by the increase in the martensitic block size inherited from the prior austenite grain structure. This study suggests that tempering at 600 °C for 2 h is an effective post-process heat treatment to homogenize the microstructure and mechanical properties of as-WAAMed 2Cr13 martensitic stainless steel.

DOI： 10.1016/j.jmrt.2022.10.077

Scopus

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Al/Cu dissimilar joining is a key technology for reducing the weight and cost of electrical components. In this study, the dissimilar friction stir lap welding (FSLW) of a Ni-containing Al alloy to pure Cu was performed, and the effects of the addition of Ni on the weld strength and interfacial microstructure were examined. A thin intermetallic compound (IMC) layer was observed at the Al/Cu weld interface produced by FSLW. The addition of 3 at.% Ni effectively improved the weld strength, although the thickness of the IMC layer increased. The IMC layer formed at the Al/Cu interface without Ni comprised CuAl<inf>2</inf> and Cu<inf>9</inf>Al<inf>4</inf> from the pure Al side. In contrast, the IMC layer formed with 3 at.% Ni consisted of (Ni,Cu)Al, CuAl, and Cu<inf>9</inf>Al<inf>4</inf> from the Al side. The addition of Ni eliminated the weak CuAl<inf>2</inf>/Cu<inf>9</inf>Al<inf>4</inf> interface, thereby improving the weld strength. The results of this study suggest that the strength of the Al/Cu weld can be effectively improved by the thinning of the IMC layer caused by FSLW and the change in interfacial microstructure caused by Ni addition.

DOI： 10.3390/met12030453

Scopus

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担当区分：筆頭著者,　責任著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.91.474

Scopus

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担当区分：筆頭著者,　責任著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.91.539

Scopus

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担当区分：最終著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.91.87

Scopus

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The anisotropic tensile behavior associated with different microstructural distributions in a thin wall of 2Cr13 martensitic stainless steel fabricated by wire arc additive manufacturing was examined. In the lower part of the thin wall, the vertical specimen had a lower strength and higher elongation than the horizontal specimen because of periodically distributed soft and hard bands of tempered martensitic structures toward the building direction. Tensile fracture preferentially occurred in the softest region when loading in the vertical direction, while inevitable crack propagation occurred in both soft and hard regions when horizontal loads were applied. In the upper part of the thin wall, no anisotropy in strength was detected, whereas significantly higher elongation was found in the vertical specimen than in the horizontal specimen. The anisotropic elongation mainly arose from the soft elongated δ-ferrite embedded in the hard quenched martensitic matrix along the building direction, because the elongated δ-ferrite served as a preferential site for crack initiation in the horizontal specimen, but not in the vertical specimen. This study suggests that heterogeneously distributed tempered martensite and elongated δ-ferrite should be appropriately controlled to diminish the anisotropy of the tensile behavior in wire arc additive manufactured martensitic stainless steel.

DOI： 10.1016/j.mtcomm.2021.102870

Scopus

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A thin wall of well-formed dense 2Cr13 martensitic stainless steel was successfully fabricated by wire arc additive manufacturing (WAAM). Heterogeneous distributions of the hardness and microstructure were found in the wall consisting of 20 layers along the building direction. The hardness profile of the thin wall mainly depended on the degree of tempering of the martensite. Based on the simulated and tested thermal history, the inter-pass temperature was lower than the martensitic transformation start (M<inf>s</inf>) temperature in the first 10 layers. The thermal effect of the subsequent passes partially tempered the quenched martensite formed in the previous pass during cooling. By contrast, the inter-pass temperature was greater than the M<inf>s</inf> temperature in the last 10 layers. The martensitic transformation mainly occurred after the final pass, resulting in a relatively homogeneous microstructure. This study suggests that the inter-pass temperature is a dominant factor in controlling the microstructural evolution during the WAAM of 2Cr13 martensitic stainless steel.

DOI： 10.1016/j.matchar.2021.111520

Scopus

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A novel strategy with shot peening (SP) pretreatment was proposed to prevent heat-affected zone liquation cracking during laser cladding of K447A nickel-based superalloy powder over its non-weldable cast structure. High dislocation density accumulated by the SP pretreatment drove the high-temperature zone of the heat-affected zone to recrystallize during the laser cladding process, as a result, the coarse columnar grain with an average dendrite stem and secondary dendrite arm width of 102.67 ± 4.24 μm was transformed into a fine equiaxed grain with an average size of 10 μm. SP could effectively inhibit liquation cracking. With the increase of SP duration, the tendency of the liquation cracking reduced. Compared with no SP BM, the total length of the liquation cracking in the longitudinal section of the laser cladding sample reduced from 1649 μm to 248.8 μm. In-situ experimental observation and process simulation were opted to study the evolution behavior of recrystallization. Recrystallization firstly developed in the region of the intergranular zone at ~1214 °C and gradually the base metal recrystallized between 1232 and 1242 °C except for the region with γ/γ′ eutectic structure. As the temperature rose further, a fine liquid film network developed in the range of 1260–1267 °C. Finally, the stress and strain-based criteria were exercised to evaluate the crack susceptibility of the liquid film. The fine liquid film network adequately constrained the liquation cracking by reducing the driving stress applied to the liquid film as well as by lowering the pressure drop of the liquid film caused by driving strain.

DOI： 10.1016/j.msea.2021.141678

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記述言語：英語   掲載種別：研究論文（学術雑誌）

Laser cladding with a paraxial powder-feeding device was used to repair as-cast K447A nickel-based superalloy, a kind of non-weldable alloy with a high Hf content and working as gas-turbine hot-section components. In the present study, the metallurgical evolution behavior of liquid film and liquation cracking mechanisms are studied. The results showed that a kind of uncommon Ni<inf>21</inf>Hf<inf>8</inf> phase was found in the incipient melting regions (IMR) of the intergranular region. In the thermal cycle, γ/Ni<inf>21</inf>Hf<inf>8</inf> eutectic liquefaction, M<inf>5</inf>B<inf>3</inf> dissolved, and γ/γ’ eutectic liquefaction successively occurred in the heating process in IMR, and liquid → γ + γ/γ’ + M<inf>5</inf>B<inf>3</inf> + γ/Ni<inf>21</inf>Hf<inf>8</inf> → γ + γ’<inf>(3)</inf> + γ/γ’ + γ/Ni<inf>21</inf>Hf<inf>8</inf> + M<inf>5</inf>B<inf>3</inf> occurred in the cooling process. The inhomogeneity of the intergranular structure leads to the liquid film with the characteristics of discontinuous distribution and uneven width. The cracking criterion of the liquid film was established based on pressure drop. When the closed liquid film meets the condition of 2γ<inf>sl</inf><1/K*(ρ<inf>s</inf>/ρ<inf>l</inf>−1*R*dt+∆h<inf>ε</inf>), the liquid film with a thickness that is greater than critical thickness will be induced into a liquation cracking.

DOI： 10.1016/j.jallcom.2020.158463

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Grain boundary engineering (GBE) has a high potential to suppress intergranular degradation and improve weld properties. Thermomechanical processing is typically used to optimize grain boundary character distributions (GBCDs) for the GBE of face-centered cubic materials with low stacking fault energy. Pre-straining plus annealing has achieved the optimal GBCD for GBE of austenitic stainless steels. Typically, cold rolling is used for pre-straining, but is not suitable for products with complex shapes. Laser peening can introduce strain locally and flexibly, even on complex parts. Therefore, this study attempted to apply laser peening as a pre-straining method during the thermomechanical processing. Several sets of process parameters during laser peening were selected to introduce suitable pre-strain into the surface of a 304 steel specimen based on previous GBE studies focusing on cold rolling plus annealing. Annealing at 1260 K for 48 h following laser peening led to an optimal GBCD with over 80 % frequency for coincidence site lattice boundaries and disconnected random boundaries. An intergranular corrosion test revealed that the laser-peened and annealed 304 steel with an optimal GBCD exhibits excellent intergranular corrosion resistance, which is far greater than that of an as-received sample and comparable to that of a cold-rolled and annealed sample.

DOI： 10.1016/j.mtcomm.2020.101572

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記述言語：英語   掲載種別：研究論文（学術雑誌）

A new solid-state bonding technique, Joule heating diffusion bonding, was used for the dissimilar bonding of commercial-purity Ti to 304 stainless steel within a short time without macroscopic deformation of the workpieces. The tensile strengths of the joints produced at various bonding parameters were examined at room temperature, and the microstructures of the joints and the fracture surfaces were analyzed to clarify the effect of the microstructural factors on the tensile strength of the joints. The tensile strength of the joints increased with the increase in the fraction of the sufficiently bonded interface. In the joints with the well-bonded interface, the tensile strength decreased with the increase in the thickness of the brittle Fe-Ti-type intermetallic compound layers at the joint interface. This study suggested that the high tensile strength could be achieved in the Joule heating diffusion bonded joints with the well-bonded interface where the thickness of the Fe-Ti-type intermetallic compound layers was thinner than 0.5 µm.

DOI： 10.3390/met10121689

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

The purpose of this study is to evaluate the corrosion resistance of weld metal by electrochemical methods and discuss the relationship between microstructure and corrosion resistance. Intergranular and pitting corrosion resistances were measured using electrochemical potentiokinetic reactivation (EPR) test and pitting potential measurement respectively. The reactivation ratio and pitting potential corresponded to its chemical composition. The specimens containing more Cr and Mo showed higher resistance. In the EPR test, the dendrite core with a relatively low Cr content was corroded. In the pitting corrosion test, Nb carbide became the initiation site of pitting corrosion which propagated along the cell structure.

DOI： 10.1016/j.corsci.2020.108867

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The effect of secondary phase formation such as MC carbide during welding on solidification cracking susceptibility of fully austenitic stainless steels with various amounts of alloying elements and carbon was investigated. The solidification cracking susceptibility of Fe-24mass%Cr-26mass%Ni stainless steels with various amounts of niobium, titanium, zirconium, and carbon was evaluated by the Trans-Varestraint test. The addition of alloying elements such as niobium, titanium, and zirconium increased the brittle temperature range (BTR). The BTRs depend on the type and combination of the alloying elements and carbon content. Titanium was the most influential element on the increase in BTR compared to the other elements. The increase in carbon content tended to decrease the BTR, especially for the specimen containing titanium. The alloying elements formed phases such as the film-like MC-type carbide (MC) phase and granular Laves phase in the specimens containing 0.05% C, and a lot of the acicular MC phases in the specimens containing 0.20% C. The start temperature of the secondary phase formation differed depending on the type of the alloying element, and the temperature tended to increase with increasing carbon content. When the additions of niobium and titanium were compared, the amount of titanium segregation during solidification was smaller due to the formation of MC phases. Solidification simulation revealed that a higher amount and faster formation of the MC phase during solidification in 2Ti–20C contributed to reducing the BTR by increasing the carbon content. Thus, it is considered that the weld solidification cracking susceptibility depended on the type and amount of formation morphology, as well as start temperature during solidification corresponding to the composition of alloying elements and carbon.

DOI： 10.1016/j.jallcom.2020.154423

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Solidification cracking has been a serious problem in austenitic stainless steels. Since solidification cracking often occurs at the solidification grain boundary, the grain boundary character distribution (GBCD) in the weld metal may affect the solidification cracking susceptibility. On the other hand, the columnar grains of the weld metal are formed by the epitaxial growth from the grains in the base metal. Therefore, it is expected that microstructure and cracking susceptibility of the weld metal can be reduced by controlling the microstructure of the base metal. In this study, the effect of microstructure of the base metal on the microstructure and solidification cracking susceptibility of the weld metal was examined. The GBCDs of the base metal of the type 310S stainless steel were changed by the thermomechanical process. The solidification cracking susceptibility was evaluated by the Trans-Varestraint test. The thermomechanically processed specimen had larger ratio of coincidence site lattice (CSL) boundaries compared with those of the as-received specimen. Moreover, the specimen was found less susceptible to solidification cracking than was the as-received material. The GBCD analysis indicated that the complex shape and distribution of the grain boundaries in the thermomechanically processed specimen affect the initiation and propagation of solidification cracks.

DOI： 10.1007/s40194-020-00865-8

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/jjws.89.509

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担当区分：筆頭著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/JJWS.90.405

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記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

DOI： 10.2207/JJWS.89.454

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

The aim of the present study was to investigate the effect of chromium and carbon in medium carbon steels on the microstructure evolution during FSW process and their mechanical property. The materials are dynamically recrystallized in austenite during FSW followed by phase transformation from austenite to ferrite, pearlite, bainite, and martensite. In the case of 0.2%C, the microstructure of the stir zone transited from pearlite and ferrite to bainite and martensite with the increase in chromium content. On the other hand, in the case of 0.4%C, the microstructure transited from pearlite to martensite with the increase in chromium content. The specimens of 0.2%C-4%Cr, 0.4%C-2%Cr and 0.4%C-4%Cr showed high tensile strength of over 1.5 GPa. In addition, even though the microstructures in the stir zones of the specimens are martensite, the elongation of 0.2%C-4%Cr was 31%while those of the others were less than 5%. TEM observation revealed that many tiny carbides precipitated in martensite of the stir zone of 0.2%C-4%Cr and micro twin formed in martensite in those of 0.4%C-2%Cr and 0.4%C-4%Cr. Thus, it was considered that auto-tempering occurred in the stir zone of 0.2%C-4%Cr during FSW process. The carbide precipitation in martensite is more effective to improve the tensile elongation compared with micro twin formation.

DOI： 10.1088/1742-6596/1270/1/012032

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

Grain boundary engineering has been attracting attention as an effective method to prevent intergranular corrosion of austenitic stainless steel and Ni based alloys. It has been considered that grain growth and recrystallization play important role in evolution of grain boundary character distribution during thermomechanical process. However, systematic researches to examine the effect of thermomechanical process parameters on grain growth and recrystallization have not been performed. In this study, grain boundary character distribution of 304 austenitic stainless steel after thermomechanical process with various parameters was analysed by electron backscatter diffraction (EBSD). Grain boundary character distribution was mainly affected by the reduction ratio of cold rolling. Abnormal grain growth was observed in the specimen with small reduction ratio (3%). Length ratio of coincident site lattice (CSL) boundaries was drastically increased to 86% in the 3% cold rolled and annealed specimens from 67% in the base material. On the other hand, normal grain growth was observed in thermomechanical processed specimens with slightly higher reduction of cold rolling (5%). In these specimens length ratio of CSL boundaries did not exceed 80%, which is required to disconnect the random boundary networks and to improve intergranular corrosion resistance effectively. Detailed analysis of grain boundary character distribution has shown that disconnection of random boundary networks was achieved by formation of annealing twins during abnormal grain growth.

DOI： 10.1088/1742-6596/1270/1/012031

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The carbon and chromium contents significantly affected the microstructure evolution in the stir zones of medium-carbon steels during the friction stir welding (FSW). The stir zone of the 0.2%C–4%Cr steel has a high strength over 1.5 GPa with a good elongation of 30.3%, while the stir zones of the 0.4%C–2%Cr and 0.4%C–4%Cr steels had ultrahigh tensile strengths over 2 GPa with low elongation. Auto-tempered martensite is the main reason for the excellent balance of strength and ductility in the stir zone of 0.2%C–4%Cr steel.

DOI： 10.1016/j.msea.2019.138060

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

The 355 aluminium alloy is known to have excellent thermodynamic characteristics that render it suitable as a raw material for rheocasting and thixoforming. However, besides the controllable transition from solid to liquid phase, the refined microstructure required in the semisolid range is one of the key factors with a strong influence on the rheology of the material. This paper intends to analyse the in situ behaviour of the microstructure, in terms of morphological change, using high-temperature laser scanning confocal microscopy. The 355 alloy was prepared via conventional casting, and refined with a 30-s exposition via ultrasonic melt treatment (UST-20 Hz, 2 kW). The material was reheated up to the thixoforming target temperature of 595 °C at which it was maintained for 0, 30, 60, 90, and 120 s, after which all the samples were cooled in water. The samples subjected to UST prior to the heat treatment were more refined in terms of microstructural evolution; they exhibited reduction in grain size (~107 ± 16 μm), smallest primary phase particle size (~81 ± 7 μm), and high circularity shape factor (~0.59 ± 0.19 μm). In situ observation methods were employed to analyse evolution mechanisms such as Ostwald ripening and coalescence.

DOI： 10.4028/www.scientific.net/SSP.285.277

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記述言語：英語   掲載種別：研究論文（学術雑誌）

The effect of alloy elements such as niobium, titanium, and zirconium on the weld solidification cracking susceptibility in fully austenitic stainless steel was investigated. Niobium, titanium, or zirconium was added as an alloy element to Fe-24 mass%Cr-26 mass%Ni stainless steel. The cracking susceptibility was evaluated by crack length, number of cracks, and brittle temperature range (BTR) corresponding to results of the Trans-Varestraint test. Depending on the addition of the alloy element, the crack length increased; the length ordering tendencies between the total crack length (TCL) and the maximum crack length (MCL) differed with the alloy addition. The BTR was obtained by corresponding the MCL to the temperature range using the measured temperature history of the weld metal and was increased by the addition of the alloy element. The maximum BTR for the specimen with titanium was 266.9°C, which was three times that of the specimen without the alloy element. The MC carbide and the Laves phase formed at the dendrite cell boundaries as secondary phases. Solidification calculation based on the Scheil model was used to investigate the effect of the type of the alloy element on the solidification temperature range. Depending on the type of the alloy element, the solidification temperature range varied. A significant difference was found between the solidification temperature range and BTR in the case of the specimen with niobium.

DOI： 10.2355/isijinternational.ISIJINT-2019-035

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担当区分：筆頭著者,　責任著者   記述言語：日本語   掲載種別：研究論文（その他学術会議資料等）

DOI： 10.2207/jjws.88.279

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